Galileo also studied natural forces, and other things that are now called principles of physics. A legend says that he climbed the Leaning Tower of Pisa, and dropped cannonballs of different weights, to see which would strike the ground first. Even though their weights were not the same, they hit the ground at the same time. Galileo found that objects fall to the ground at the same rate, unless things like wind resistance change the rate. This went against the views of Aristotle, an ancient philosopher whose theory was different. Galileo's findings were ignored by most people, and Aristotle's view was still accepted as correct until Isaac Newton proved Galileo was right. This also led to Newton creating his Law of Gravity.

Galileo also tried to determine the speed of light. He climbed a hill, and had an assistant climb another hill, both carrying lanterns with closed shutters. He then opened the shutter of his lantern. His assistant opened his own shutter when he saw Galileo's lantern. Galileo then measured the time it took for his assistant's shutter to open. Knowing the time difference, and the distance between the hills, he tried to estimate the speed of light. However, this did not work.[5]

Galileo came to accept the findings of Copernicus, that the Sun was the center of the then-known universe, and not the Earth. Because he promoted this and other ideas, he came to the notice of the Committee of Propaganda, the dreaded Inquisition. The Church taught that the Earth stood still, while everything in the sky moved around it. The Inquisition ruled that other theories could only be discussed as possibilities, not facts.

Galileo later defended his views in his most famous work, Dialogue Concerning the Two Chief World Systems, published in 1632. The book was in the form of conversations between three men. The man representing the Church's point of view was called 'Simplicio'. At this, the Inquisition took action. He was arrested and put on trial. They found him "vehemently suspect of heresy". They reminded him of the fate of Giordano Bruno, who had been burnt at the stake for heresy. Bruno's heresy was to believe the Earth went round the Sun, and that there were many other stars. The Inquisition forced Galileo to recant (say he was wrong) under the threat of execution, and to withdraw his works from publication. Galileo spent the rest of his life under house arrest.[6][7]

In time, more and more of Galileo's findings were accepted as true. Late in the 20th century, Pope John Paul II called Galileo the "father of modern physics", and made a public apology for how the Church treated him earlier.

Dialogo sopra i due massimi sistemi del mondo, Tolemaico et Copernicano. Florence, 1632. The famous 'dialogue between two world systems'.

Discorsi et demonstrazioni mathematiche, intorno a due nuove scienze. (discourses and mathematical demonstrations relating to two new sciences) Leiden 1638. This, on the mathematics of motion, was not such a controversial subject, and was published in Holland, out of the Inquisition's territory.